How Carbon-neutral E-fuels Work

Audi's A3 Sportback TCNG vehicle -- and the concept of powering it with synthetic methane -- seems revolutionary, but it builds on a long history. As soon as a reliable natural gas transmission system became established in the early 1930s, road warriors had the idea to burn it in their cars. In fact, at a technical level, vehicles powered by gasoline and natural gas work on the same principles: Fuel is mixed with air in the cylinder of a four-stroke engine and then ignited by a spark plug to move a piston up and down (see How Natural Gas Vehicles Work for more details). Had the United States and the Middle East not discovered large crude oil fields and a vast supply of cheap, readily available fuel, natural gas vehicles might have become much more popular.

Today, NGVs represent just a tiny fraction of the world automobile market. There are about 120,000 NGVs driving on American roads and more than 15 million worldwide at the end of 2011. Five countries -- Iran, Pakistan, Argentina, Brazil and India -- accounted for almost 70 percent of the global market share in 2011 [source: Natural Gas Vehicles for America]. NGVs are successful in these regions because the areas lack the capacity to refine oil. But more nations will embrace natural gas in the next 10 years, especially as gasoline prices continue to rise. That's because natural gas tends to cost much less than gasoline. By some estimates, the fuel savings associated with NGVs run about 30 percent less, on average, than gasoline-powered vehicles [source: Consumer Reports].

Keep Reading Below

Fuel costs are only one reason NGVs are so attractive -- they also produce far fewer exhaust emissions. According to the U.S. Environmental Protection Agency (EPA), NGVs reduce carbon monoxide by 70 to 90 percent, nitrogen oxides by 75 to 95 percent and carbon dioxide by 20 to 30 percent [source: Natural Gas Vehicles for America]. Add all of this up, and you have a car that has far less impact on air quality and slows the progress of global warming. You have, in fact, the cleanest internal-combustion vehicle on the planet, which is an honor the EPA has bestowed upon the natural-gas version of the Honda Civic.

It's not all birdsong and green meadows, though. Natural gas vehicles cost more to purchase and present refueling issues that almost rival the recharging woes of EVs. There were only about 1,000 NGV fueling stations in the United States as of December 2011, and only half of those were open to the public [source: NGV Global]. You can install a home refueling system, but it adds even more up-front ownership costs. And, of course, you can't escape the fact that NGVs, while they produce far fewer CO2 emissions, still contribute to the warming of the planet.

As a transitional technology, however, natural gas vehicles are an important piece of the puzzle, especially if you can run them on synthetic methane made from carbon dioxide. Remember, that's what Audi will do in its Werlte plant: split water into hydrogen and oxygen via electrolysis, then combine the hydrogen gas with carbon dioxide obtained from a biogas plant. But wait a minute. Doesn't electrolysis require electricity? And doesn't electricity require a power plant? Funny you should ask, because that's the last element of Audi's e-gas project.

Four Rings and Two Fuel Tanks

If you're a carmaker hell-bent on making synthetic methane, you benefit greatly if you have a natural gas vehicle to receive the fuel. For the brand with the four rings, that car is the A3 Sportback TCNG. Interestingly, Audi decided not to build a "pure" NGV -- the A3 TCNG carries both natural gas and gasoline storage tanks, and the engine shifts seamlessly between the two. If you're low on fuel and a CNG fueling station is nowhere to be found, you can still drive the car on gasoline. That may seem like a dubious compromise, but the idea is to run the vehicle in natural gas mode as much as possible. Audi calculates that 1,500 specially modified A3 TCNG models can each drive 9,320 miles (15,000 kilometers) per year on renewably generated e-gas [source: Audi Worldwide]. Want to be one of the people logging those miles? Audi plans to begin selling the A3 Sportback TCNG in 2013.